In 2018 I am tutoring a 2 day ant identification workshop at Preston Montford Field Centre near Shrewsbury. Details from the link below;
These are some of the resources I find useful for identifying ant specimens from Britain especially, but also Europe.
Volume 24 of Naturalists' handbooks, ISSN 0962-6360.
Gary J. Skinner, Geoffrey W. Allen
Company of Biologists, 1996.
Out-dated, new species have been added and names changed since 1996, but still the most recently published book on the identification of British ants. The keys to workers and queens are over-simplified and can be misleading, the male ant key is probably best avoided altogether.
The RES Handbook for the identification of British insects; Vol 6 Part 3c. Hymenoptera - Formicidae. Barry Bolton & Cedric A. Collingwood.
Is available as a download from the RES website on the out of print handbooks page. Again needs updating, but the keys are still useful if the changes to taxonomy since publication are taken into account. Especially useful for males.
The Formicidae (Hymenoptera) of Fennoscandia and Denmark, C. A. Collingwood, Fauna Entomologica Scandinavica, vol. 8, 1979.
Published a few years after the RES key so similar comments apply. Useful keys and line-drawings. Still in print, and also available as a PDF from AntWeb.
Fourmis d'Europe Occidentale, Claude Lebas, Christophe Galkowski, Rumsaïs Blatrix, Philippe Wegnez. Delachaux et Nestlé, 2016.
Keys to, and descriptions of, all the ant genera and many of the species/species groups of Western Europe.
Identification and information resources.
There is lots of useful information and links for British Hymenoptera in general on the BWARS website.
Especially useful for ants are the Stenamma and Myrmica sabuleti/scabrinodis/speciodes guides on the download page.
Key to Identifying Common French Ants. Has a dichotomous key to the majority of French species, so covers all native British species and many others.
AntWeb Huge online database of high resolution photos of ants for identification, and more.